The present invention relates to nonlinear optical loop mirror devices, and is directed more particularly to nonlinear optical loop mirror devices which include optical fibers having dispersions which decrease monotonically along the length thereof.
Nonlinear optical loop mirrors (NOLMs) and related nonlinear amplifying loop mirrors (NALMs) have developed into important building blocks which are widely used in the switching, shaping and other processing of optical pulses. In such loop mirrors, optical pulses are coupled into a loop of optical fiber through a coupler that divides them into two component pulses which propagate around the loop in opposite directions, and which are transmitted and/or reflected by the loop mirror, depending upon the phases with which the component pulses return to the coupler. A NOLM of this general type is described in "Nonlinear Optical Loop Mirror", by N. Doran and D. Wood, Optical Letters, Vol. 13, No. 1, pp. 56-58, January 1988. A NALM of this general type is described in "Nonlinear Optical Loop Mirror", by M. Fermann, et al., Optics Letters, Vol. 15, No. 13, pp. 752-754, July 1990.
If a coupler divides an input pulse into two equal component pulses, and if the loop affects these component pulses in the same way, i.e., symmetrically, the component pulses will interfere constructively on their return to the coupler and, consequently, will be reflected back through the coupler port through which they entered. If the pulses are divided into unequal component pulses, and/or if the loop affects the component pulses differently, i.e., unsymmetrically or asymmetrically, the pulses may interfere either constructively, destructively or partly constructively and partly destructively. In such cases the pulses returning to the coupler may be reflected, transmitted or partly reflected and partly transmitted. This directional routing property of asymmetric NOLMs provides great opportunities for signal processing which are not provided by symmetrical NOLMs.
Asymmetrical NOLMs differ from one another primarily in the methods or structures that are used to make them asymmetrical. One approach to introducing asymmetry into a loop is to couple an input pulse into the loop with a power-coupling ratio that differs from 50:50. One example of a NOLM that uses this form of asymmetry is described in the above-cited Doran and Wood articles.
Other approaches to introducing as asymmetry into a loop include locating rotated sections of birefringent fiber therein, or positioning an optical amplifier asymmetrically therein. An example of the former approach is described in "Optical Switching Using Fiber Ring Reflectors", J. Moores, et al., J. Opt. Soc. Am. B, Vol. 8, No. 3, pp. 594-601, March 1991. An example of the latter approach is described in the above-cited Fermann, et al. article.
Another known building block of optical fiber systems includes optical fibers that have dispersions that vary along the length thereof. These fibers can be used to effect the shapes of pulses if two conditions are met. First, the pulse wavelength needs to be greater than the zero dispersion wavelength (i.e., the anomalous dispersion regime). Second, the pulse intensity needs to be sufficiently high to generate self phase modulation. Pulses under these conditions are or will rapidly become what are known as optical solitons. The balance between dispersion and self-phase modulation is the defining mechanism for solitons. When the rate of change of dispersion in the fiber is equal to the attenuation rate of the fiber, the dispersion/SPM balance continues and the pulse width is constant. When the rate of change of dispersion is greater than the attenuation rate, the pulse width can slowly compress. An example of an optical fiber having such a variable dispersion is described in "Compensation of Soliton Broadening in Nonlinear Optical Fibers With Loss," K. Tajima, Optics Letters, v. 12, no. 1, p. 54-56, 1987.
Optical fibers having dispersions which decrease in the direction of propagation are commonly referred to as dispersion decreasing (DD) fibers, while those having dispersions which increase in the direction of propagation are known as dispersion increasing (DI) fibers. As explained in "A Single-Mode Fiber with Chromatic Dispersion Varying Along the Length", V. Bogatyrev, et al., Journal of Lightwave Technology, Vol.9, No. 5, pp. 561-566, May 1991, such fibers may be produced by changing the axial dopant concentration of the fiber, the diameter of the fiber core, or other fiber parameters. A particularly advantageous way of producing DD or DI fibers is described in copending, commonly assigned U.S. patent application Ser. No. 08/172,937, filed Dec. 27, 1993 now U.S. Pat. No. 4,504,829, which is hereby expressly incorporated herein by reference.
Prior to the present invention, DD and DI fibers were used primarily as pulse shaping devices, e.g., pulse compressors and decompressors. They also found use in maintaining the shape of optical solitons in long optical fiber waveguides having attenuations that were too large to be neglected. An application of the latter type is described in "Dramatically Improved Transmission of Ultra-Short Solitons Through 40 km of Dispersion Decreasing Fiber", A. J. Stentz, R. W. Boyd and A. F. Evans, Optics Letters, Vol. 20, no. 17, pp 1770-1772, Sept., 1995.
Prior to the present invention, however, DD and DI fibers have not been used to introduce directional asymmetry into NOLMs and NALMs. As a result, NOLMs and NALMs have not taken advantage of the many opportunities that DD and DI fibers create for using NOLMs and NALMs to perform new optical functions or to perform known optical functions in new and better ways.